In the process of making semiconductor devices photoresists and antireflective materials are applied to a substrate. Photoresists are photosensitive films used to transfer an image to a substrate. A photoresist is formed on a substrate and then exposed to a radiation source through a photomask (reticle). Exposure to the radiation provides a photochemical transformation of the photoresist, thus transferring the pattern of the photomask to the photoresist. The photoresist is then developed to provide a relief image that permits selective processing of the substrate.
Photoresists are typically used in lithographic structures to create features such as vias, trenches or combination of the two, in a dielectric material. In such a process, the reflection of radiation during exposure of the photoresist can limit the resolution of the image patterned in the photoresist due to reflections from the material beneath the photoresist. Reflection of radiation from the substrate/photoresist interface can also produce variations in the radiation intensity during exposure, resulting in non-uniform linewidths. Also, unwanted scattering of radiation expose regions of the photoresist not intended, which again results in linewidth variation. The amount of scattering and reflection will vary from one region of the substrate to another resulting in further linewidth variation.
With recent trends towards high-density semiconductor devices, there is a movement in the industry to use low wavelength radiation sources into the deep ultraviolet light (300 nm or less) for imaging a photoresist, e.g., KrF excimer laser light (248 nm), ArF excimer laser light (193 nm), excimer laser light (157 nm), electron beams and soft x-rays. However, the use of low wavelength radiation often results in increased reflections from the upper resist surface as well as the surface of the underlying substrate.
Substrate reflections at ultraviolet and deep ultraviolet wavelengths are notorious for producing standing wave effects and resist notching which severely limit critical dimension (CD) control. Notching results from substrate topography and non-uniform substrate reflectivity which causes local variations in exposure energy on the resist. Standing waves are thin film interference or periodic variations of light intensity through the resist thickness. These light variations are introduced because planarization of the resist presents a different thickness through the underlying topography. Thin film interference plays a dominant role in CD control of single material photoresist processes, causing large changes in the effective exposure dose due to a tiny change in the optical phase. Thin film interference effects are described in “Optimization of optical properties of resist processes” (T. Brunner, SPIE 10 Proceedings Vol. 1466, 1991, 297).
Bottom anti-reflective coatings (BARCs) have been used with photoresists to reduce thin film interference with some success. However, these relatively thin absorbing BARCs have fundamental limitations. At times, the photoresist does not provide sufficient resistance to subsequent etching steps to enable effective transfer of the desired pattern to a material, e.g., a dielectric, beneath the photoresist. The photoresist is consumed after transferring the pattern into the underlying BARC and substrates. In addition, the trend to smaller sub 90 nm node feature sizes requires the use of relatively thin photoresists (>200 nm) to avoid image collapse. If a substantial etching depth is required, or if it is desired to use certain etchants for a given underlying material, the photoresist thickness is now insufficient to complete the etch process. Consequently, the photoresist does not effectively transfer the desired pattern into the underlying substrate or antireflective material.
The present trend to 248 nm, 193 nm and 157 nm lithography and the demand for sub 200 nm features requires that new processing schemes be developed. To accomplish this, tools with higher numerical aperture (NA) are emerging. The higher NA allows for improved resolution but reduces the depth of focus of aerial images projected onto the photoresist. Because of the reduced depth of focus, a thinner photoresist is required. However, as the thickness of the photoresist is decreased, the photoresist becomes less effective as a mask for subsequent dry etch image transfer to the underlying substrate. Without significant improvement in the etch resistance exhibited by current single material photoresists, these systems cannot provide the necessary etch characteristics for high resolution lithography.